Conventionally, a vacuum system is to utilized widely in semiconductor application, in particular in the process of thin film, dry etching, ion implanting and lithography. In the vacuum technology, a vacuum pump always plays a key role to the vacuum system. Especially in a multistage pump, the precision of the pump has much to do with the assembling way. The object is to keep the clearance control the axial of the position small enough.
U.S. Pat. No. 5,013,227 describes a multistage Roots pump, wherein the rotor is fixed by clamping screws. This makes the precision of axial clearance controlled perfectly, which enables highly accurate assembly to be achieved. It still has many disadvantages. For example, the structure is relatively complicated, thereby involving high cost. Another prior art have also been developed. The shaft and the rotor completed integral with each other and the compression chamber is formed by two symmetrical housings. Such a pump is easy to assemble, but the precision of the clearance is hard to control.